In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, photovoltaic (PV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
In the case of OFETs, the performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
In prior art various materials have been proposed for use as organic semiconductors in OFETs, including small molecules like for example pentacene, and polymers like for example polyhexylthiophene. However, the materials and devices investigated so far do still have several drawbacks, and their properties, especially the processability, charge-carrier mobility, on/off ratio and stability do still leave room for further improvement.
Such charge transport materials also find important application in OLEDs. For example, WO2004/084260 describes a hole transporting layer that is introduced between the anode and the emissive layer in a polymer OLED as interlayer. Such an interlayer can significantly improve the performance, especially the lifetime of the OLEDs. For optimum performance of such an OLED device, a high charge carrier mobility and appropriate energy alignment between interlayer, anode and emissive layer are required. In WO2004/084260, polymers based on triarylamine are suggested for use as interlayer. However, these polymers have the disadvantage that the energy level of the highest occupied molecular orbital cannot be adjusted in range that is sufficiently wide to fit the different emissive polymers used in the emissive layer, because the HOMO (highest occupied molecular orbital) of such polymers is mainly determined by the triarylamine unit.
One aim of the present invention is to provide new organic semiconducting materials for use in electronic devices, especially for use as active materials in OFETs or interlayers in light emitting devices, which have advantageous properties, in particular good processability, high charge-carrier mobility, high on/off ratio in case of OFETs, high oxidative stability and long lifetime in electronic devices. Another aim is to extend the pool of semiconducting materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
It has been found that these aims can be achieved by providing semiconducting materials as claimed in the present invention. These materials are based on copolymers comprising one or more tetrasubstituted cis- or trans-indenofluorene units or derivatives thereof
wherein R denotes aromatic or aliphatic hydrocarbyl groups, and two adjacent groups R may also form a spiro group with the respective fluorene group to which they are attached,and further comprising one or more thiophene or selenophene unit.
In particular, it has been found that such copolymers are suitable for use as semiconducting materials in electronic devices like transistors and OLEDs, as they have good processibility and at the same time show a surprisingly high charge carrier mobility and high oxidative stability.
WO 2004/041901 describes polymers comprising aryl-substituted indenofluorenes and further units like triarylamine or heteroaryl moieties, and their use in OLED or OFET devices. Surin et al., Adv.Funct.Mat. 2005, 15, 1426-1434 disclose copolymers of 6,6-12,12-tetraoctyl trans-indenofluorene and dithiophene, terthiophene or quaterthiophene units that are optionally alkylated. Sonar et al., Macromolecules 2004, 37, 709-715 discloses copolymers of 6,6-12,12-tetraoctyl trans-indenofluorene and 4-hexyldithieno[3,2-b:2′,3′-e]pyridine. However, copolymers as claimed in the present invention are not disclosed.
Thiophenes have good hole transport ability. However, they have only moderate solubility in many organic solvents, which does negatively affect their processability in a film-forming process and leads to films with moderate uniformity. Indenofluorenes, on the other hand, are soluble in conventional organic solvents and thus show good processability, enabling the formation of films with high uniformity. However, they have been mainly suggested to be efficient electron transporting and light-emitting moieties. It was therefore surprising that a copolymer according to the present invention, wherein indenofluorene units are combined with hole transporting moieties like thiophenes, shows high charge carrier mobility and enables the preparation of electronic devices like transistors with high on/off ratios and OLEDs using these polymers as hole transporting layer or interlayer.